All green plants synthesize their food through the process of photosynthesis and water is an essential component required by the leaves for photosynthesis.
Xylems carry water from the roots to the other parts of the plant. Leaves play a vital role in this biological process whereas stem transports the prepared food to different parts of the plant. Root system plays a significant role in the plant’s survival as they help in transferring water and other minerals. Hence, various parts of plants help in carrying out daily life processes.
Like all living organism, plants too require an excretory system to discharge excess wastes from their body and it is the crucial life process as photosynthesis and transportation.
What is Transpiration?
Transpiration is the biological process by which water is lost in the form of water vapour from the aerial parts of the plants.
During the process of transpiration, water molecules are carried through plants tissues from roots to the small pores on the underside of leaves called stomata, where it changes to vapour and is released to the atmosphere.
Types of Transpiration
There are three different types of transpiration:
Stomatal transpiration is the evaporation of water from the stomata of the plants. Most of the water from the plants is transpired this way. The water near the surface of the leaves changes into vapour and evaporates when the stomata are open.
Lenticles are minute openings in the bark of branches and twigs. Evaporation of water from the lenticles of the plants is known as lenticular transpiration.
Lenticles are not present in all the plants. A very small amount of water is lost through lenticles.
It is the evaporation of water from the cuticle of the plants. It is a waxy covering on the surface of the leaves of the plants. About 5-10% of the water from the leaves is lost through cuticular transpiration. During dry conditions when the stomata are closed, more water is transpired through the cuticles.
Also Read: Guttation
Factors affecting Transpiration
Different factors affecting the transpirate rate are:
The cellular factors affecting the rate of transpiration are:
- Orientation of leaf,
- The water status of the plant,
- Structural Peculiarities of leaf,
- Total number and distribution of stomata in a leaf.
The environmental factors affecting the rate of transpiration are:
- Atmospheric pressure,
- Wind speed or velocity.
Let us have a detailed look at the various factors affecting transpiration:
The amount of water vapour present in the air at a particular time and temperature. The rate of transpiration is inversely proportional to relative humidity. More the relative humidity less is the transpirate rate.
A high temperature lowers the relative humidity and opens the stomata even in darkness. As a result, the rate of transpiration increases.
The stomata open during the day and close in the dark. Presence of light is directly proportional to the rate of transpiration.
If the air is still, the transpiration rate is low. This is because the water vapour accumulates around the transpiring organs and reduce the diffusion pressure deficit of the air.
If the air is moving the saturated air around the leaves is removed and the transpiration rate increases.
The transpiration rate is directly proportional to the absorption of water by the roots from the soil. A decrease in water absorption causes the closure of stomata and wilting, thereby, reducing the rate of transpiration.
Surface Area of the Leaves
A leaf having more surface area will show more transpiration rate than the leaf with a lesser surface area.
Ascent of Sap
When water evaporates through the leaves, a pull is created through the xylem, and water moves back to the leaves. This is known as the transpiration pull.
The ascent of sap that is driven by transpiration depends on the following properties of water:
- Cohesion – This is the mutual attraction between molecules of water.
- Adhesion – The attraction of water molecules towards polar surfaces.
- Surface tension – The molecules of water are more attracted to each other in the liquid phase than in the gas phase.
Opening and Closing of Stomata
Stomata consist of a pair of guard cells with an aperture in between. It remains open during the daytime and is closed at night. The reason for the opening and closing of this structure is the turgidity of guard cells.
The interior wall of the guard cells present towards the aperture is dense and flexible. The stomata open when the turgidity of the guard cells increases. The exterior walls bulge out and the interior walls form a crescent shape.
The orientation of the microfibrils in the guard cells also plays an important role in the opening of the stomata. The radial orientation of the microfibrils makes it easier for the stomata to open. The stomata close when the turgidity of the guard cells decreases due to the water loss and the interior walls form a crescent shape retrieve their original shape.
In dicots, the lower side of leaves have more stomata while in monocots, both the sides have an equal number of stomata.
Also Read: Water Transport
To learn more about transpiration and its process, keep visiting BYJU’S or download BYJU’S app for further reference.